This invention relates to radar seekers used in guided missiles and more particularly to antenna systems for radar seekers operating at dual frequency bands.
As is known in the art, a reflector antenna generally includes a feed circuit and at least one conductive member generally referred to as a reflector. The feed circuit radiates RF energy at the reflector and the reflector directs the RF energy in a desired direction. Reflector antennas are used in those applications requiring an electrically large antenna having a high gain characteristic. In order to allow positioning of the feed element and electronics in a more convenient location, a dual reflector antenna system is sometimes used. One type of dual reflector antenna system is generally referred to as a Cassegrain reflector antenna.
A Cassegrain reflector antenna typically includes a first or main reflector having a parabolic shape with an aperture centrally disposed therein. A second, or subreflector having a hyperbolic shape is placed between the vertex of the main reflector and the prime focus of the main reflector. The precise location of the subreflector relative to the main reflector may be selected to provide an antenna having preselected electrical characteristics. A feed, generally referred to as a Cassegrain feed, is disposed in the aperture of the main reflector. In a transmit mode, the feed radiates electromagnetic energy at the subreflector. In a preferred situation, the subreflector intercepts substantially all of the electromagnetic energy and reflects such energy back toward the main reflector. The main reflector intercepts substantially all of the electromagnetic energy fed from the subreflector and reflects such electromagnetic energy in a desired direction. The geometrical arrangement of the parabolically shaped main reflector and the hyperbolically shaped subreflector are selected such that electromagnetic signals (or rays) reflected by the main reflector will be parallel.
As is also known, an array antenna includes a plurality of antenna elements disposed in an array in a manner wherein the radio frequency signals emanating from each of the plurality of antenna elements combine with constructive interference in a desired direction. In radar guided missiles, missile seeker antennas are often disposed on a gimbal. It is desirable in radar guided missiles to provide missile seeker antennas having polarimetric receive properties and which operate in dual frequency bands. In a radar guided missile application, a dual band dual polarized antenna must share a common radiating aperture to provide antenna radiation characteristics with high directivity and relatively low sidelobe levels at both bands. To operate in dual frequency bands, it is often necessary to provide two antenna assemblies and dispose such assemblies in the shared aperture. It is increasingly more difficult to dispose two antenna assemblies in a shared aperture in the small diameter of a missile.